1. Field of the Invention
This invention relates to printing heads for ink jet printers and more particularly to a drop-on-demand printing head.
2. Brief Description of the Prior Art
Many varieties of ejection systems for ink drop printers have been devised in an effort to achieve higher speed of operation while providing uniform dimensions of the ink droplets. It is important that the ink droplets be precisely controlled in size and ejected with sufficient velocity that the imprint on the medium is sharp and positioned with great accuracy.
A pulsed drop ejection (drop-on-demand) system is functionally comparable to a subminiature reciprocating pump, although such print heads have frequently been considered from the standpoint of an acoustic system. Wave actions of the ink and resonances of the ink chamber analyzed on that basis, while ignoring the ink flow resulting from the pump action, can be misleading. Moreover, in a drop-on-demand system, the ink chamber is in a stable condition until the first droplet is ejected. If this is followed by the ejection of a series of droplets, a generally continual flow of ink through the chamber is necessary. This imparts the characteristics of a pump. In this construction, however, the usual piston-cylinder combination is replaced by a piezoelectric transducer acting on the ink in the chamber.
When a voltage pulse of proper polarity is applied to the transducer causing the piezoelectric material to expand radially, the resulting sudden decrease in volume of the ink chamber creates pressure that disrupts the meniscus at the ejection orifice causing the ejection of a droplet of ink. When the voltage pulse is reversed resulting in the expansion of the ink chamber, the meniscus immediately reforms and, if the increase in chamber volume takes place relatively slowly, by a more gradual change in the pulse voltage, the meniscus acts as a check valve preventing air from entering the chamber and allowing replen-ishment of the ink in the chamber through a feed line from the ink reservoir. The repetition of the ejection cycle must allow sufficient time for the chamber to reach substantially the identical starting condition as for the previous drop. Under these conditions, droplets of identical size and velocity are ejected.
U.S. Pat. No. 3,857,049 to Zoltan discloses a drop-on-demand print head using a transducer in which the perimiter of a disk of piezoelectric material projects into the ink chamber, which is in the form of an annular chamber around the disk. Seals to confine the ink in the chamber are formed by a pair of O-rings that engage the upper and lower surfaces of the piezoelectric disk. The edge portion of the piezoelectric disk is in direct contact with the ink. The O-rings are resilient and therefore limit the drop repetition speed.
Other types of print heads are illustrated by U.S. Pat. Nos. 2,512,743 to Hansell, and 4,387,383 to Sayko.